Thanks for all the inputs guys! I've run duel battery's before just wondered about this setup. I passed it up
Aside for the engine, the single most productive thing you can have on a Jeep is Dual Batteries.
Alternator dies, No Problem, you have enough battery reserve to get someplace you can get an alternator.
One battery dies, No Problem, you have a second, just unplug the dead battery and continue as normal.
Have a winch? Inverter for 110v power? 12 volt powered tools? Electric air compressor?
No problem, you have LOTS of battery reserve to do the jobs you need to.
Need to weld something?
Need to Lend Power to someone with a dead battery?
Need a battery in camp for fans, radio, ect.?
No problem with high current quick disconnects on one or both of the batteries.
Need to get that 'Bucket' started in REALLY COLD weather?
Nothing like a second battery kicking in and cranking the
out of that engine!
Second battery, second starter relay ($15 to $20) on the second battery makes everything but the starter motor REDUNDANT in the starting circuit.
No more 'Solenoid' (Starter Relay) problems,
You probably won't know one starter relay failed until you do maintenance...
No more being stranded by a $15 'Made In China' failure prone part...
With a $10 common automotive relay/socket added,
You can ISOLATE your 'Emergency' battery,
In the event something shorts out and drains the 'Primary' battery...
No worries about the 'Kids' leaving something turned on, or forgetting to turn the head lights off...
------------
The batteries you choose are going to make the system more redundant, while giving you more useful energy.
With TWO batteries, the load is split 50/50 between TWO batteries,
So your Deep Cycle batteries will live longer...
Deep Cycle batteries are made to SLOWLY be Charged/Discharged,
So guys that RAPIDLY Charge/Discharge a single Deep Cycle are damaging that battery.
With TWO batteries, neither battery gets charged or discharged quickly, so they live longer.\
-----
'STARTING' batteries are designed to charge/discharge quickly,
BUT NOT DEEPLY DISCHARGED, which will damage them.
A second 'Starting' battery will divide the load between two batteries,
So you don't damage the single Starting battery.
-----
My personal favorite is a 'Marine Starting' battery.
It has qualities of both 'Starting' and 'Deep Cycle', while not exactly being either.
Heavy built for rough service,
Capable of recovering from fairly deep discharge,
And capable of charging faster without damage than a true Deep Cycle,
A matched pair of these are good choice for what I do with my Jeep.
Everyone will have to assess the APPLICATION for their own vehicles,
I'm just saying that after 40 years of experimentation and assessment of parts,
This is the most PRODUCTIVE & RELIABLE system I've found so far, USING COMMON PARTS that can be found in any parts store or junk yard...